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  • 1
    ISSN: 0168-9002
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Physics Letters B 230 (1989), S. 159-161 
    ISSN: 0370-2693
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We have developed a high-resolution ac susceptometer that uses a rf superconducting quantum interference device to directly measure the flux coupled into a superconducting detection coil from a sample's changing magnetic moment in an applied ac field. The system operates in a frequency range from 0.01 to 1500 Hz and an applied ac field range of 0.1–400 μT with a sensitivity of about 5×10−12 A m2 for magnetic moment measurement, and at a reduced sensitivity down to 0.001 Hz. The instrument is based on an existing dc magnetometer system and uses that system's temperature control and dc superconducting magnet to allow operation over a temperature range from 2 to 400 K and in applied dc fields of ±5.0 T. During a measurement all operations are controlled automatically by computer from a menu-driven software system, with user input required only on initiation of a measurement sequence. Both real and imaginary components of the ac susceptibility can be determined.
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  • 4
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 374 (1995), S. 769-769 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] SIR - Global emission of isoprene (2-methyl-l, 3-butadiene) from vegetation is estimated at 3x10 g yr"1, similar to methane1. Because isoprene reacts very rapidly with hydroxyl radicals, it plays an important role in atmospheric chemistry2"4. Why plants emit isoprene has been a mystery since ...
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  • 5
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The design, construction, and performance of a Cu nuclear refrigerator are reported. The first nuclear stage (total 275 moles Cu with 104 moles Cu in 8 T) has refrigerated a195Pt NMR thermometer in the low-field experimental region to 15 µK; it can keep experiments below 20 µK for more than 1 week. It can alternatively precool a second nuclear stage (2 moles Cu in 9 T). Demagnetizing this stage has resulted in a temperature of at most 12 µK as measured by another195Pt NMR thermometer attached to the stage. The details of the thermometry are described and possible origins of the observed internal heat leaks as well as unexpected contributions to the specific heat of the nuclear stages are discussed.
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 23 (2000), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Isoprene emission from plants is highly temperature sensitive and is common in forest canopy species that experience rapid leaf temperature fluctuations. Isoprene emission declines with temperature above 35 °C but the temperature at which the decline begins varies between 35 and 44 °C. This variability is caused by the rate at which leaf temperature is increased during measurement with lower temperatures associated with longer measurement cycles. To investigate this we exposed leaves of red oak (Quercus rubra L.) to temperature regimes of 35–45 °C for periods of 20–60 min. Isoprene emission increased during the first 10 min of high temperature exposure and then decreased over the next 10 min until it reached steady state. This phenomenon was common at temperatures above 35 °C but was not noticeable at temperatures below that. The response was reversible within 30 min by lowering leaf temperature to 30 °C. Because there is no storage of isoprene inside the leaf, this behaviour indicates regulation of isoprene synthesis in the leaf. We demonstrated that the variability in isoprene decline results from regulation and explains the variability in the temperature response. This is consistent with our theory that isoprene protects leaves from damage caused by rapid temperature fluctuations.
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  • 7
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Predicting the environmental responses of leaf photosynthesis is central to many models of changes in the future global carbon cycle and terrestrial biosphere. The steady-state biochemical model of C3 photosynthesis of Farquhar et al. (Planta 149, 78–90, 1980) provides a basis for these larger scale predictions; but a weakness in the application of the model as currently parameterized is the inability to accurately predict carbon assimilation at the range of temperatures over which significant photosynthesis occurs in the natural environment. The temperature functions used in this model have been based on in vitro measurements made over a limited temperature range and require several assumptions of in vivo conditions. Since photosynthetic rates are often Rubisco-limited (ribulose, 1-5 bisphosphate carboxylase/oxygenase) under natural steady-state conditions, inaccuracies in the functions predicting Rubisco kinetic properties at different temperatures may cause significant error. In this study, transgenic tobacco containing only 10% normal levels of Rubisco were used to measure Rubisco-limited photosynthesis over a large range of CO2 concentrations. From the responses of the rate of CO2 assimilation at a wide range of temperatures, and CO2 and O2 concentrations, the temperature functions of Rubisco kinetic properties were estimated in vivo. These differed substantially from previously published functions. These new functions were then used to predict photosynthesis in lemon and found to faithfully mimic the observed pattern of temperature response. There was also a close correspondence with published C3 photosynthesis temperature responses. The results represent an improved ability to model leaf photosynthesis over a wide range of temperatures (10–40 °C) necessary for predicting carbon uptake by terrestrial C3 systems.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Mesophyll conductance (gm) generally correlates with photosynthetic capacity, although the causal relationship between the two is unclear. The response of gm to various CO2 regimes was measured to determine its relationship to environmental changes that affect photosynthesis. The overall effect of CO2 growth environment on gm was species and experiment dependent. The data did not statistically differ from the previously shown A–gm relationship and was unaffected by CO2 treatment. The consequences of the CO2 effect on gm for interpreting photosynthesis in individual cases were investigated. Substantial effects of assumed versus calculated gm on leaf properties estimated from gas-exchange measurements were found. This differential error resulted in an underestimation in ratio of maximum carboxylation to electron transport, especially in plants with high photosynthetic capacity. Including gm in the calculations also improved the agreement between maximum carboxylation rates and in vitro Rubisco measurements. It is concluded that gm is finite and varies with photosynthetic capacity. Including gm when calculating photosynthesis parameters from gas-exchange data will avoid systematic errors.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 21 (1998), S. 0 
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Isoprene emission from leaves is temperature dependent and may protect leaves from damage at high temperatures. We measured the temperature of white oak (Quercus alba L.) leaves at the top of the canopy. The largest short-term changes in leaf temperature were associated with changes in solar radiation. During these episodes, leaf temperature changed with a 1 min time constant, a measure of the rate of temperature change. We imposed rapid temperature fluctuations on leaves to study the effect of temperature change rate on isoprene emission. Leaf temperature changed with a 16 s time constant; isoprene responded more slowly with a 37 s time constant. This time constant was slow enough to cause a lag in isoprene emission when leaf temperature fluctuated rapidly but isoprene emission changed quickly enough to follow the large temperature changes observed in the oak canopy. This is consistent with the theory that isoprene functions to protect leaves from short periods of high temperature. Time constant analysis also revealed that there are two processes that cause isoprene emission to increase with leaf temperature. The fastest process likely reflects the influence of temperature on reaction kinetics, while the slower process may reflect the activation of an enzyme.
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  • 10
    ISSN: 1573-5079
    Keywords: gas exchange ; optoelectric ; feedback ; light ; LED ; instrumentation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Light-emitting diodes (LED) can provide large fluxes of red photons and so could be used to make lightweight, efficient lighting systems for photosynthetic research. We compared photosynthesis, stomatal conductance and isoprene emission (a sensitive indicator of ATP status) from leaves of kudzu (Pueraria lobata (Willd) Ohwi.) enclosed in a leaf chamber illuminated by LEDs versus by a xenon arc lamp. Stomatal conductance was measured to determine if red LED light could sufficiently open stomata. The LEDs produced an even field of red light (peak emission 656±5 nm) over the range of 0–1500 μmol m-2 s-1. Under ambient CO2 the photosynthetic response to red light deviated slightly from the response measured in white light and stomatal conductance followed a similar pattern. Isoprene emission also increased with light similar to photosynthesis in white light and red light. The response of photosynthesis to CO2 was similar under the LED and xenon arc lamps at equal photosynthetic irradiance of 1000 μmol m-2 s-1. There was no statistical difference between the white light and red light measurements in high CO2. Some leaves exhibited feedback inhibition of photosynthesis which was equally evident under irradiation of either lamp type. Photosynthesis research including electron transport, carbon metabolism and trace gas emission studies should benefit greatly from the increased reliability, repeatability and portability of a photosynthesis lamp based on light-emitting diodes.
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